U.S. patent number 4,874,569 [Application Number 07/002,048] was granted by the patent office on 1989-10-17 for ceramic composite and methods of making the same.
This patent grant is currently assigned to Lanxide Technology Company, LP. Invention is credited to Christopher R. Kennedy, Jack A. Kuszyk.
United States Patent |
4,874,569 |
Kuszyk , et al. |
October 17, 1989 |
Ceramic composite and methods of making the same
Abstract
There is provided a method for producing a self-supporting
ceramic composite comprising (1) a ceramic matrix obtained by
oxidation of an aluminum zinc alloy to form a polycrystalline
oxidation reaction product of the metal with an oxidant, and (2)
one or more fillers embedded by the matrix. The metal alloy and
permeable mass of filler having at least one defined surface
boundary are oriented relative to each other so that formation of
the oxidation reaction product will occur into said mass of filler
and in a direction towards said defined surface boundary. On
heating the metal to a first temperature above its melting point
but below the melting point of said oxidation reaction product to
form a body of molten parent metal, the molten metal reacts with
said oxidant to form said oxidation reaction product which
infiltrates said mass of filler to said defined surface boundary.
The resulting infiltrated mass is heated to a second temperature in
order to remove or oxidize at least a substantial portion of any
residual non-oxidized metallic constituents from or in said
infiltrated mass without substantial formation of said oxidation
reaction product beyond said defined surface boundary, thereby
producing a self-supporting ceramic composite.
Inventors: |
Kuszyk; Jack A. (Newark,
DE), Kennedy; Christopher R. (Newark, DE) |
Assignee: |
Lanxide Technology Company, LP
(Newark, DE)
|
Family
ID: |
21699019 |
Appl.
No.: |
07/002,048 |
Filed: |
January 12, 1987 |
Current U.S.
Class: |
264/82; 501/153;
501/127 |
Current CPC
Class: |
B22D
41/32 (20130101); C04B 35/652 (20130101) |
Current International
Class: |
B22D
41/32 (20060101); B22D 41/22 (20060101); C04B
35/65 (20060101); C04B 035/71 (); C04B
035/02 () |
Field of
Search: |
;501/87,88,94,96,98,92,119,127,128,134,153,154
;423/345,412,618,625,411 ;264/56,59,60,66,65 ;75/235,230,232 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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0116809 |
|
Aug 1984 |
|
EP |
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0155831 |
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Sep 1985 |
|
EP |
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0169067 |
|
Jan 1986 |
|
EP |
|
851765 |
|
Nov 1985 |
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GR |
|
Other References
"Oxidation of Molten Aluminum Alloys. Reaction with
Refractories"--M. Drouzy and M. Richard--Mar., 1974--Fonderie,
France No. 332, pp. 121-128. .
"Refractories for Aluminum Alloy Melting Furnaces"--B. Clavaud and
V. Jost--Sep., 1980--Lillian Brassinga (from French), Jan.
1985..
|
Primary Examiner: Dixon, Jr.; William R.
Assistant Examiner: Griffis; Andrew
Attorney, Agent or Firm: Mortenson; Mark G. Boyer; Michael
K. McShane; William E.
Claims
What is claimed is:
1. A method for producing a self-supporting ceramic composite
comprising (1) a ceramic matrix obtained by oxidation of a parent
metal comprising an aluminum alloy to form a polycrystalline
material comprising an oxidation reaction product of the parent
metal with at least one oxidant; and (2) at least one filler
embedded by the matrix, which method comprises:
(a) positioning a parent metal, comprising an aluminum alloy having
at least about 1% by weight zinc, adjacent to a permeable mass of
filler having at least one defined surface boundary and orienting
said parent metal and said filler relative to each other so that
formation of an oxidation reaction product of the parent metal with
an oxidant will occur into said mass of filler and in a direction
towards said defined surface boundary;
(b) heating said parent metal to a first temperature above its
melting point but below the melting point of said oxidation
reaction product to form a body of molten parent metal and reacting
the molten parent metal with said oxidant at said first temperature
to form said oxidation reaction product, and at said first
temperature maintaining at least a portion of said oxidation
reaction product in contact with and extending between said body of
molten metal and said oxidant, to draw molten metal through the
oxidation reaction product towards the oxidant and towards and into
the adjacent mass of filler so that fresh oxidation reaction
product continues to form within the mass of filler at an interface
between the oxidant and previously formed oxidation reaction
product, and continuing said reaction for a time sufficient to
infiltrate said mass of filler to said defined surface boundary,
with said ceramic matrix, said ceramic matrix containing at least
some residual non-oxidized metallic constituents of said parent
metal; and
(c) heating the resulting infiltrated mass of step (b) in at least
one environment selected from the group consisting of an
oxygen-containing atmosphere, an inert atmosphere and a vacuum to a
second temperature above the first temperature but below the
melting point of the oxidation reaction product to remove or
oxidize at least a substantial portion of said residual
non-oxidized metallic constituents of said parent metal without
substantial formation of oxidation reaction product beyond said
defined surface boundary, thereby producing a self-supporting
ceramic composite.
2. The method of claim 1, wherein at least one dopant in addition
to zinc is used in conjunction with the parent metal.
3. The method of claim 1 or claim 2, wherein said filler comprises
from about 3% by weight to about 10% by weight silica.
4. The method of claim 1 or claim 2, wherein said oxidant comprises
an oxygen-containing gas and said oxidation reaction product
comprises an oxide of aluminum.
5. The method of claim 4, wherein said oxidant comprises air at
atmospheric pressure.
6. The method of claim 1 or claim 2, wherein said first temperature
is from about 850.degree. C. to about 1450.degree. C.
7. The method of claim 1, wherein said first temperature is from
about 950.degree. C. to about 1100.degree. C.
8. The method of claim 1, wherein said second temperature is
greater than about 1250.degree. C.
9. The method of claim 1, wherein said second temperature is at
least about 1400.degree. C.
10. The method of claim 1, wherein heating step (c) to said second
temperature is effected in air at atmospheric pressure.
11. The method of claim 1 or claim 2, wherein said filler comprises
at least one metal oxide, boride, nitride, or carbide of a metal
selected from the group consisting of aluminum, cerium, hafnium,
lanthanum, silicon, neodymium, praseodymium, samarium, scandium,
thorium, uranium, titanium, yttrium, and zirconium.
12. The method of claim 1 or claim 2, wherein said filler comprises
a material selected from the group consisting of granules, fibers,
tubes, refractory fiber cloth, and mixtures thereof.
13. The method of claim 1 or claim 2, wherein said filler comprises
at least one of alumina and silicon carbide.
14. The method of claim 1 or claim 2, wherein said ceramic matrix
resulting from said heating step (c) comprises interconnected
porosity having at least a portion being accessible from at least
one surface of said ceramic composite.
15. The method of claim 14, wherein said interconnected porosity
comprises openings having a mean diameter of less than about 6
microns.
16. A method for producing a self-supporting ceramic composite
comprising (1) a ceramic matrix obtained by oxidation of a parent
metal comprising an aluminum alloy to form a polycrystalline
material comprising an oxidation reaction product of the parent
metal with at least one oxidant; and (2) at least one filler
embedded by the matrix, which method comprises:
(a) positioning a parent metal, comprising an aluminum alloy having
about 4-7% by weight zinc, adjacent to a permeable mass of filler
having at least one defined surface boundary and orienting said
parent metal and said filler relative to each other so that
formation of an oxidation reaction product of the parent metal with
an oxidant will occur into said mass of filler and in a direction
towards said defined surface boundary;
(b) heating said parent metal to a first temperature above its
melting point but below the melting point of said oxidation
reaction product to form a body of molten parent metal and reacting
the molten parent metal with said oxidant at said first temperature
to form said oxidation reaction product, and at said first
temperature maintaining at least a portion of said oxidation
reaction product in contact with and extending between said body of
molten metal and said oxidant, to draw molten metal through the
oxidation reaction product towards the oxidant and towards and into
the adjacent mass of filler so that fresh oxidation reaction
product continues to form within the mass of filler at an interface
between the oxidant and previously formed oxidation reaction
product, and continuing said reaction for a time sufficient to
infiltrate said mass of filler to said defined surface boundary,
with said ceramic matrix, said ceramic matrix containing at least
some residual non-oxidized metallic constituents of said parent
metal; and
(c) heating the resulting infiltrated mass of step (b) in at least
one environment selected from the group consisting of an
oxygen-containing atmosphere, an inert atmosphere and a vacuum to a
second temperature above the first temperature but below the
melting point of the oxidation reaction product to remove or
oxidize at least a substantial portion of said residual
non-oxidized metallic constituents of said parent metal without
substantial formation of oxidation reaction product beyond said
defined surface boundary, thereby producing a self-supporting
ceramic composite.
17. The method of claim 16, wherein said first temperature is from
about 850.degree. C. to about 1450.degree. C.
18. The method of claim 16, wherein said first temperature is from
about 950.degree. C. to about 1100.degree. C.
19. The method of claim 16, wherein said second temperature is
greater than about 1250.degree. C.
20. The method of claim 16, wherein said second temperature is at
least about 1400.degree. C.
21. The method of claim 16, wherein said ceramic matrix resulting
from said heating step (c) comprises interconnected porosity having
at least a portion being accessible from at least one surface of
said ceramic composite.
22. The method of claim 21, wherein said interconnected porosity
comprises openings having a mean diameter of less than about 6
microns.
23. The method of claim 1 or claim 2 wherein said filler comprises
at least one whisker.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention broadly relates to novel ceramic composites
and methods of making the same. In a more specific aspect, the
invention relates to ceramic composites particularly useful as
refractories, such as steel plant refractories. The invention also
relates to methods of making the ceramic composites by the directed
oxidation at elevated temperatures of a parent metal into a
permeable mass of filler material followed by a subsequent heating
step to remove or oxidize residual non-oxidized metal
constituents.
2. Description of Commonly Owned Patent Applications and
Background
The subject matter of this application is related to Marc S.
Newkirk et al. U.S. Pat. No. 4,713,360 which issued on Dec. 15,
1987 and was based on commonly owned U.S. patent application Ser.
No. 818,943, filed Jan. 15, 1986, which is a continuation-in-part
of Ser. No. 776,964, filed Sept. 17, 1985, which is a
continuation-in-part of Ser. No. 705,787, filed Feb. 26, 1985,
which is a continuation-in-part of Ser. No. 591,392, filed Mar. 16,
1984, and entitled "Novel Ceramic Materials and Methods for Making
the Same". This patent discloses the method of producing
self-supporting ceramic bodies grown as the oxidation reaction
product from a parent metal precursor. Molten parent metal is
reacted with a vapor-phase oxidant to form an oxidation reaction
product, and the metal migrates through the oxidation reaction
product toward the oxidant thereby continuously developing a
polycrystalline ceramic body of the oxidation reaction product. The
ceramic body can be produced having metallic components and/or
porosity, which may or may not be interconnected. The process may
be enhanced by the use of an alloyed dopant, such as in the case of
an aluminum parent metal oxidized in air. This method was improved
by the use of external dopants applied to the surface of the
precursor metal as disclosed in commonly owned and copending U.S.
patent applications Ser. No. 822,999, filed Jan. 27, 1986, which is
a continuation-in-part of Ser. No. 776,965, filed Sept. 17, 1985,
which is a continuation-in-part of Ser. No. 747,788, filed June 25,
1985, which is a continuation-in-part of Ser. No. 632,636, filed
July 20, 1984, all in the name of Marc S. Newkirk et al., and
entitled "Methods of Making Self-Supporting Ceramic Materials".
The subject matter of this application is also related to that of
commonly owned and copending U.S. patent applications Ser. No.
819,397, filed Jan. 17, 1986, which is a continuation-in-part of
Ser. No. 697,876, filed Feb. 4, 1985, both in the name of Marc S.
Newkirk et al. and entitled "Composite Ceramic Articles and Methods
of Making the Same". These applications disclose a novel method for
producing self-supporting ceramic composites by growing an
oxidation reaction product from a parent metal into a permeable
mass of filler, thereby infiltrating the filler with a ceramic
matrix.
Further developments of the foregoing methods enable the formation
of ceramic composite structures which (1) contain therein one or
more cavities which inversely replicate the geometry of a shaped
precursor parent metal, and (2) have a negative pattern which
inversely replicates the positive pattern of a parent metal
precursor. These methods are described, respectively, (1) in
Commonly Owned U.S. patent application Ser. No. 823,542 filed Jan.
27, 1986, in the name of Marc S. Newkirk et al, entitled "Inverse
Shape Replication Method of Making Ceramic Composite Articles and
Articles Obtained Thereby", and (2) in Commonly Owned U.S. patent
application Ser. No. 896,157, filed Aug. 13, 1986, in the name of
Marc S. Newkirk, and entitled "Method of Making Ceramic Composite
Articles with Shape Replicated Surfaces and Articles Obtained
Thereby".
Also, methods of making ceramic composite structures having a
pre-selected shape or geometry were developed. These methods
include the utilization of a shaped preform of permeable filler
into which the ceramic matrix is grown by oxidation of a parent
metal precursor, as described in Commonly Owned U.S. patent
application Ser. No. 861,025, filed May 8, 1986, in the name of
Marc S. Newkirk et al. and entitled "Shaped Ceramic Composites and
Methods of Making the Same". Another method of making such shaped
ceramic composites includes the utilization of barrier means to
arrest or inhibit the growth of the oxidation reaction product at a
selected boundary to define the shape or geometry of the ceramic
composite structure. This technique is described in Commonly Owned
U.S. patent application Ser. No. 861,024, filed May 8, 1986, in the
name of Marc S. Newkirk et al. and entitled "Method of Making
Shaped Ceramic Composites with the Use of a Barrier".
The entire disclosures of all of the foregoing Commonly Owned
Patent Applications and Patent are expressly incorporated herein by
reference.
Common to each of these Commonly Owned Patent Applications and
patent is the disclosure of embodiments of a ceramic body
comprising an oxidation reaction product, most typically
interconnected in three dimensions, and, optionally, one or more
non-oxidized constituents of the parent metal or voids or both. The
metal phase and/or the voids may or may not be interconnected
depending largely on such factors as the temperature at which the
oxidation reaction is allowed to proceed, the composition of the
parent metal, the presence of dopant materials, etc. For example,
if the growth process is continued to substantially exhaust
(convert) the metal constituents, porosity will result as a partial
or nearly complete replacement of the metal phase throughout the
bulk of the composite body, while developing a dense ceramic skin
at the surface of the composite body. In such a case, the
interconnected porosity is typically accessible from the surface of
the ceramic body from which matrix development initiated.
Ceramic refractories are useful as components for applications
requiring good resistance to thermal shock, corrosion and erosion
in contact with molten metals. Such components may, for example, be
used in control means for regulating the flow of molten metals in
molten metal transfer systems, for example, in the manufacture and
handling of steel. Such uses include, for example, slide gates,
sub-entry nozzles, and ladle shrouds. Slide gates are used for
controlling the flow of molten metal from a ladle. Generally, slide
gate systems including some rotary designs, consist of a fixed
nozzle attached to and within a movable plate. The flow of molten
metal from a ladle is controlled by moving the movable plate to
fully or partially align openings. When filling the ladle and
during shut-off, the openings are misaligned. The principal
advantage of the slide gate system over a conventional stopper rod
system is its improved reliability of shutoff, ability to modulate
molten metal flow, and lack of aspiration of the molten steel
product stream. However, even the best slide gate systems, such as
high-alumina slide gate systems, are inadequate for certain molten
metals, such as specialty steel like low-carbon, high-manganese
grades. These corrosive steel compositions will seriously attack
the bonding media used in most high-alumina grade slide gate
systems.
Today, in the U.S. market, the majority of the slide gate
refractories are composed of either tar-impregnated high-alumina,
or fired magnesia materials. However, such slide gate refractories
do not possess the thermal shock, corrosion and erosion resistance
criteria to stand up to long ladle holding and teeming times and
preheating, and therefore have a short service life.
The ceramic composites of this invention offer potential for use as
steel plant refractories such as slide gate refractories, that do
not have the foregoing deficiencies while still possessing thermal
shock, corrosion and erosion resistance criteria to withstand long
ladle holding and teeming times and preheating. In addition, they
may be useful for other applications requiring thermal shock
resistance and high temperature strength retention.
SUMMARY OF THE INVENTION
In accordance with the present invention, there is provided a
method for producing a self-supporting ceramic composite comprising
(1) a ceramic matrix obtained by oxidation of a parent metal
comprising an aluminum-zinc alloy to form a polycrystalline
material consisting essentially of an oxidation reaction product of
the parent metal with an oxidant, and (2) a filler embedded by the
matrix.
Generally, a precursor metal and permeable mass of filler are
oriented relative to each other so that the growth of a
polycrystalline material resulting from the oxidation of a
precursor metal (hereinafter referred to as the "parent metal" and
defined below) as described in the above-referenced Commonly Owned
Patent Applications is directed towards and into a permeable mass
of filler material. (The terms "filler" and "filler material" are
used herein interchangeably.) The mass of filler has at least one
defined surface boundary and is infiltrated with polycrystalline
material to the defined surface boundary to provide a ceramic
composite. Under the process conditions of this invention, the
molten parent metal oxidizes outwardly from its initial surface
(i.e., the surface exposed to the oxidant) towards the oxidant and
into the mass of filler by migrating through its own oxidation
reaction product. The oxidation reaction product grows into the
permeable mass of filler. This results in novel ceramic matrix
composites comprising a matrix of a ceramic polycrystalline
material embedding the filler materials.
The parent metal used in the ceramic matrix growth process
comprises an aluminum alloy having at least about 1% by weight
zinc, and this parent metal is heated to a first temperature above
its melting point but below the melting point of the oxidation
reaction product thereby forming a body or pool of molten parent
metal which is reacted with an oxidant, preferably a vapor-phase
oxidant, e.g., air, to form the oxidation reaction product. At this
first temperature or within this first temperature range, the body
of molten metal is in contact with at least a portion of the
oxidation reaction product which extends between the body of molten
metal and the oxidant. Molten metal is drawn through the oxidation
reaction product towards the oxidant and towards and into the mass
of filler material to sustain the continued formation of oxidation
reaction product at the interface between the oxidant and
previously formed oxidation reaction product. The reaction is
continued for a time sufficient to infiltrate the filler material
to the defined surface boundary with the oxidation reaction product
by growth of the latter, which has therein inclusions of
non-oxidized metallic constituents of the parent metal.
The resulting ceramic composite comprises a filler and a ceramic
matrix which is a polycrystalline oxidation reaction product and
contains residual non-oxidized constituents of the parent metal,
most typically aluminum and zinc but also may include other metals
as well. The ceramic composite is heated to a second temperature
(or within this second temperature range) above the first
temperature, but below the melting point of the oxidation reaction
product, in order to remove or oxidize at least a substantial
portion of the residual non-oxidized metallic constituents, as by
volatilization or oxidation of the metal constituents, from the
polycrystalline material without substantial formation of the
oxidation reaction product beyond the defined surface boundary.
Heating to this second temperature may be carried out either in
vacuum, an inert atmosphere, or more preferably, an
oxygen-containing atmosphere or, most preferably, air. Some of the
removed metal phase is replaced essentially by porosity or voids.
Other metal phases are oxidized in situ, converting the metal to an
oxidized species. The final structure comprises a ceramic matrix
and filler material, and the ceramic matrix consists essentially of
oxidation reaction product and interconnected porosity with at
least a portion being accessible from one or more surfaces of the
ceramic composite. Preferably, the surface porosity is
characterized by openings having a mean diameter of less than about
6 microns, which prevents the penetration of some materials such as
molten steel.
The products of the present invention are essentially ceramic; that
is, essentially inorganic and substantially void of metal, although
there may be some inclusions or islands of metal. The products are
adaptable or fabricated for use as refractories, which, as used
herein, are intended to include, without limitation, industrial
slide gate valve refractories that slidably contact the bottom
portion of a vessel, ladle or the like, containing molten metal,
such as steel, to permit and regulate the flow of molten metal
through an aperture in the ladle.
As used in this specification and the appended claims, "oxidation
reaction product" means the product of reaction of metals with an
oxidant thereby forming an oxide compound.
As used herein and in the claims, "oxidant" means one or more
suitable electron acceptors or electron sharers and may be a solid,
a liquid or a gas (vapor), or some combination of these at the
process conditions.
The term "parent metal" as used in this specification and the
appended claims refers to that aluminum alloy metal having
typically at least about 1 to 10 percent by weight zinc and which
is the precursor of the polycrystalline oxidation reaction product
and includes that aluminum alloy metal, and commercially available
aluminum alloy metal having typically at least about 1 to 10
percent by weight zinc, as well as impurities and/or alloying
constituents therein.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic, cross-sectional view in elevation showing an
assembly of an aluminum alloy parent metal, overlaying filler
material and a support bed contained in a refractory crucible;
and
FIG. 2 is a partial schematic, vertical cross-sectional view
showing a slide gate valve, slidably disposed between a top plate
of the bottom portion of a ladle and a tube holder that holds a
tube through which molten metal passes after leaving the ladle.
DETAILED DESCRIPTION OF THE INVENTION
Referring to the drawings for the practice of the present
invention, in FIG. 1 a parent metal 10, comprising an aluminum
alloy having at least about 1 to about 10 percent by weight zinc,
is formed into an ingot, billet, rod, plate or the like. This body
of parent metal 10 and a permeable mass of filler material 12
having at least one defined surface boundary 14 are positioned
adjacent to each other and oriented with respect to each other so
that growth of the oxidation reaction product will be into the
filler material 12 and in a direction towards the defined surface
boundary 14 in order that the filler material 12, or a part
thereof, will be infiltrated by the growing oxidation reaction
product. The parent metal 10 and filler material 12 are embedded in
a suitable support material 16 substantially inert under the
process conditions and of such constituency so that oxidation
reaction will not proceed into this bedding, and the upper or
exposed surface of the mass of filler is flush with the surface of
the bedding. Suitable bedding materials include, for example,
certain grades of particulate alumina such as 38 Alundum
manufactured by Norton Company. The assembly or lay-up is contained
in a suitable refractory vessel or crucible 18.
The filler material 12 preferably comprises ceramic or refractory
material and may be a lattice or array of a bed of particulates,
granules, powders, aggregate, refractory fiber cloth, fibers,
tubes, tubules, pellets, whiskers, or the like, or a combination of
the foregoing. The array or arrangement of filler material(s) 12
may be either loose or bonded and has interstices, openings,
intervening spaces, or the like, to render it permeable to the
oxidant and to the oxidation reaction product growth. Further,
suitable filler(s) depending upon specific end use of the product,
may include for example, metal oxides, borides, nitrides, or
carbides of a metal selected from the group consisting of aluminum,
cerium, hafnium, lanthanum, silicon, neodymium, praseodymium,
samarium, scandium, thorium, uranium, titanium, yttrium, and
zirconium. Certain of these fillers may require protective coatings
to prevent their reaction and/or oxidation under the conditions of
the process. In one embodiment of the invention, the filler
includes from about 3 percent to 10 percent by weight of silica,
such as in combination with alumina. Alumina filler found
especially useful has a mesh size of from about 5 to 500 (U.S.
standard sieve). Silicon carbide as filler may have a mesh size of
from about 500 to about 1000 (U.S. standard sieve).
The assembly is, in any case, arranged so that growth of the
oxidation reaction product will occur into the filler material 12
such that void space between filler particles will be substantially
filled by the grown oxidation reaction product. A matrix of the
polycrystalline material resulting from the oxidation reaction
product growth is simply grown into and/or around the filler
material 12 so as to embed and infiltrate the latter preferably to
its defined surface boundary 14 without substantially disturbing or
displacing the filler material 12. Thus, no external forces are
involved which might damage or disturb the arrangement of the
filler material 12 and no awkward and costly high temperature, high
pressure processes and facilities are required as in known
conventional processes to achieve a dense composite ceramic
structure. In addition, the stringent requirements of chemical and
physical compatibility necessary for pressureless sintering to form
ceramic composites are greatly reduced or eliminated by the present
invention.
A solid, liquid, or vapor-phase oxidant, or a combination of such
oxidants may be employed. Vapor-phase oxidants include, without
limitation, oxygen, oxygen-argon, or other inert gas mixtures and
air.
Solid oxidants include reducible oxides such as silica, tin oxide,
or zinc oxide. When a solid oxidant is employed, it is usually
dispersed through the entire bed of filler or through a portion of
the bed adjacent to the parent metal, in the form of particulates
admixed with the filler, or perhaps as coatings on the filler
particles.
If a liquid oxidant is employed, the entire bed of filler or a
portion thereof adjacent to the molten metal is coated or soaked as
by immersion in the oxidant to impregnate the filler. A suitable
liquid oxidant includes low melting glasses.
Zinc as a dopant material (which is described below in greater
detail) promotes or facilitates growth of the oxidation reaction
product and subsequent removal of the non-oxidized metallic
constituents from the oxidation reaction product initially formed.
The zinc dopant is alloyed into the aluminum parent metal, and
comprises from about 1 percent by weight to about 10 percent by
weight, and preferably about 4 percent to about 7 percent by
weight. Additional dopant materials (as disclosed in the
aforementioned. Commonly Owned Patent Application and Patent may be
used in conjunction with the parent metal 10 as by alloying dopant
material with the parent metal 10, applying an external coating to
the surface of the parent metal 10, or by incorporating or mixing
the dopant materials with the filler material(s) 12. For example,
magnesium may be used to augment the dopant action of zinc.
Referring to FIG. 1, a body of aluminum parent metal 10 along with
the mass of permeable filler material 12 are positioned in a
crucible or other refractory container 18 such that at least one
metal surface of the parent metal 10 is exposed to the adjacent to
or surrounding mass of filler material 12. If a vapor-phase oxidant
is used, the mass of filler is permeable to the gaseous oxidant
present in the oxidizing atmosphere (typically air at ambient
atmospheric pressure). The resulting assembly is then heated to a
first temperature range in the presence of the oxidant in a
suitable furnace (not shown in the drawings) to elevate the
temperature thereof in the region, typically, with air as the
oxidant, between about 850.degree. C. to about 1450.degree.C., or
more preferably, between about 950.degree. C. to about 1100.degree.
C. to form a pool or body of molten parent metal. The temperature
region depends upon the filler material 12, dopant or dopant
concentrations, oxidant, or the combination of any of these. At
this temperature region parent metal transport begins to occur
through the oxide skin normally protecting the aluminum parent
metal.
The continued high temperature exposure of the parent metal 10 to
the oxidant allows the continued oxidation of parent metal 10 to
form a polycrystalline oxidation reaction product of increasing
thickness. This growing oxidation reaction product progressively
infiltrates the permeable mass of filler material 12 with an
interconnected oxidation reaction product matrix which also may
contain non-oxidized parent metal constituents, thus forming a
cohesive composite. The growing polycrystalline matrix impregnates
or infiltrates the filler material 12 at a substantially constant
rate (that is, a substantially constant rate of thickness increase
over time), provided there is a relatively constant source of
oxidant, for example, by allowing a sufficient interchange of air
(or oxidizing atmosphere) in the furnace. Interchange of oxidizing
atmosphere, in the case of air, can be conveniently provided by
vents in the furnace. Growth of the matrix continues for a time
sufficient for the polycrystalline oxidation reaction product to
infiltrate the mass of filler material 12 to the defined boundary
14, which preferably occurs when substantially all of the parent
metal 10 is consumed, i.e., substantially all of the parent metal
10 has been converted into the matrix.
The ceramic composites initially produced by the oxidation of the
aluminum alloy parent metal with the oxidant comprises the filler
material(s) infiltrated and embedded, preferably to the defined
boundary, with the polycrystalline oxidation reaction product of
the parent metal with the oxidant, and one or more non-oxidized
metallic constituents of the parent metal including aluminum and
zinc, and other metals depending on the parent metal composition.
The volume percent of residual metal (non-oxidized metallic
constituents) can vary over a wide range depending on whether or
not the oxidation reaction process is conducted largely to exhaust
aluminum alloy parent metal. By way of example only, a ceramic
composite formed from aluminum alloy metal and 50 volume percent
filler processed in air at about 1000.degree. C. may contain about
0.5 to 10 volume percent residual metal.
In order to produce a ceramic composite substantially devoid of
metallic constituents, such as for a composite used for slide gate
valve refractories, the non-oxidized metallic constituents
(residual metal) present after the first heat treatment are
substantially removed and/or oxidized in situ by a second or
subsequent heating step. The initially formed ceramic composite is
heated at a temperature higher than the temperature first employed
in forming the initial ceramic composite. This second heating step
may be accomplished by elevating the temperature to effect the
substantial volatilization and/or oxidation of the residual metal.
This second heating step may be carried out in an oxygen-containing
or inert atmosphere or in a vacuum. An oxygen-containing atmosphere
is preferred because removal of residual metal by oxidation thereof
can be effected at a lower temperature than removal by
volatilization in an inert atmosphere or in a vacuum. Air at
ambient atmospheric pressure is most preferred for reasons of
economy.
The assembly is heated in the furnace in the presence of the
desired atmosphere to elevate the temperature thereof in the region
typically between about 1250.degree. C. to about 2000.degree.C.;
more preferably at least about 1400.degree.C., or from about
1400.degree. C. to about 1600.degree.C. This temperature is higher
or above the temperature that was employed to produce the initially
formed ceramic composite. At these elevated temperatures, any
residual non-oxidized metallic constituents of the aluminum alloy
parent metal are essentially removed or converted to an oxide
without any further growth beyond the defined surface boundary. It
is believed that a majority of the residual non-oxidized metallic
constituents are essentially helped to be removed through
volatilization of the zinc dopant. Some of the residual aluminum
metal will oxidize in situ without effecting the defined boundary
of the part. The zinc dopant not only promotes or facilitates
growth of the oxidation reaction product, but volatilizes at
elevated temperature, generating porosity and high surface area
which then promotes oxidation of residual non-oxidized metallic
constituents of the aluminum alloy parent metal leaving minimal
residual metal in the composite.
As was previously mentioned, the amount of zinc that is to be
alloyed into the aluminum parent metal preferably comprises from
about 4 percent by weight to about 7% by weight (based on the
weight of the aluminum parent metal labelled as 10). The zinc may
be alloyed directly with unalloyed commercial purity aluminum,
e.g., of 99%, 99.5% or 99.7% grade. If so desired, high or super
purity aluminum, e.g., 99.9% or purer, may be used as a base for
the alloying addition. This may be desirable where the refractory
end-product is to be used in conjunction with very high purity
molten metals where even traces of contaminants are unwanted. On
the other hand, certain zinc-containing commercial wrought alloys,
e.g., of the Aluminum Association 7000 series or casting alloys,
e.g., of the Aluminum Association 700 series may be used where the
zinc content is above 1.0%, preferably above 4.0%, and where the
presence of other alloying elements is not harmful to the end use.
For example, alloy 7021 which contains 5.0-6.0% zinc, 1.2-1.8%
magnesium, 0.08-0.18 % zirconium with permitted maxima for the
following elements: silicon 0.25%; iron 0.40%; copper 0.25%;
manganese 0.10%; chromium 0.05%; titanium 0.10%; other elements
each 0.05%; up to a total of 0.15% (all percentages by weight) the
balance being aluminum, is one among several such alloys which
would comprise a suitable parent metal for the invention. In this
case, the magnesium present in the alloy augments the dopant action
of zinc.
When desired, the composite may be cooled and removed from the
furnace. The cooled body may then be machined. (e.g., such as by
milling, polishing, grinding or the like) on one or more surfaces
to desired tolerances. This alternative may be particularly
desirable in the manufacture of ceramic articles requiring close
tolerances.
In one preferred embodiment of the present invention, displayed in
FIG. 2 the ceramic composites of the invention can be fabricated
for use as slide gate valve refractories. The slide gate valve,
generally illustrated as 20 in FIG. 2, contacts a top plate 22 or
the bottom portion of a ladle, generally illustrated as 24,
containing molten metal 26 (i.e., molten steel). Top plate 22 is
integrally bound to the ladle 24 and has a top plate aperture 28
which is in direct communication with a ladle aperture 30 disposed
in the bottom of the ladle 24. The slide gate valve 20 has a slide
gate structure 32 with at least one slide gate aperture 34. A drive
means 36, such as a throttling cylinder, or the like, is coupled to
the slide gate 20 to slide (or rotate) the slide gate along the
bottom surface of the top plate 22 to either align or misalign the
slide gate aperture 34 with the top plate aperture 28 and the ladle
aperture 30. A tube holder means, generally illustrated as 40,
holds a tube 38 and supports the slide gate valve 20, the top plate
22, and the ladle 24 that is bound to the top plate 22. Tube 38
conducts the flow of molten metal 26 after the same leaves ladle 24
through slide gate 20. If the slide gate valve refractory 20 is
disposed by the drive means 36 such that the aperture 34 of the
slide gate valve refractory 20 is totally misaligned with top plate
aperture 28 and with ladle aperture 30 of the ladle 24, molten
metal 26 will not flow from the ladle 24. Also, molten metal 26 (as
will be explained in greater detail hereinafter) will not penetrate
into and through the porosity of the ceramic matrix in the
structure 32 of the slide gate valve 20. As depicted in FIG. 2 by
the label 34 which is connected to a dotted line, when the slide
gate valve 20 is slidably positioned along the top plate 22 and the
bottom portion of the ladle 24 such that the slide gate aperture 34
is generally aligned with the top plate aperture 28 and with ladle
aperture 30 of the ladle 24, molten metal 26 will flow by gravity
from the ladle 24 through the respective apertures into the tube
38.
The slide gate structure 32 must be extremely flat, i.e., to within
tolerances of 1/2000 of an inch or less, and must be held tightly
against the bottom surface of the top plate 22 so that molten metal
will not leak out between the contacting surfaces. The slide gate
structure 32, as well as the structure of the top plate 22, is
composed of refractory materials or components that are capable of
being machined (such as by milling, grinding, polishing, or the
like) extremely smooth so the structure of the top plate 22 and the
structure 32 of the slide gate valve 20 cannot pull out the grains
of the other during opening and closing of the slide gate valve 20
with the coupled drive means 36. The structure 32 of the slide gate
valve 20 should not have pores which are too large since molten
metal would penetrate the pores and weaken the structure 32.
Furthermore, the slide gate structure 32 must possess extremely
good thermal shock resistance and must be composed of refractory
materials or components that are strong enough to resist chemical
corrosion and erosive effects from flowing molten metal
compositions. In order to fabricate a slide gate structure 32 from
a ceramic composite possessing the foregoing properties and/or
criteria, the ceramic composite should contain a ceramic matrix
substantially consisting essentially of non-metallic and inorganic
material(s). Any substantial amount of non-oxidized metallic
constituents within a ceramic composite, such as aluminum, could be
detrimental to the performance of the material by lowering its high
temperature strength, possibly exhibiting oxidation overgrowth
beyond the slide gate dimensions and causing the gate components to
bond together, as well as affecting thermal shock performance.
Hence, the slide gate valve 20 would fail in its function or have
to be replaced after minimal use, most likely due to spalling,
cracking, or surface overgrowth.
The ceramic composite structure obtained after removing and/or
oxidizing substantially all of the residual non-oxidized metallic
constituents of the aluminum parent metal is a coherent ceramic
composite typically having from about 5% to about 98% by volume of
the total volume of the composite structure comprised of one or
more of the filler material embedded within a polycrystalline
ceramic matrix. The polycrystalline ceramic matrix is comprised of
about 94.5% or more by weight (of the weight of polycrystalline
oxidation reaction product) of interconnected alpha-alumina, about
5% or less of zinc aluminate, and about 0.5% or less by weight of
non-oxidized metallic constituents of the aluminum parent
metal.
The polycrystalline ceramic matrix exhibits some porosity ranging
from about 2% by volume to about 25% by volume of polycrystalline
ceramic matrix, preferably not more than about 10%. It is believed
that some porosity is required in order to provide the desired
thermal shock resistance of the refractory product. At least a
portion of the porosity is accessible from the surface, and
typically about 5% of such porosity have pore openings whose
diameter measures from about 1 micron to about 8 microns.
Preferably, the openings of the porosity accessible from the
surface have a mean diameter of about 6 microns or less, where 6 is
the mean of a normal Gaussian distribution curve. An alumina-based
ceramic composite having openings on its surface that measure about
6 microns or less in diameter is particularly useful in fabricating
a slide gate refractory since molten steel will not penetrate its
structure.
The ceramic composite structure of this invention possesses the
following properties: a three-point bend test for hot Modulus of
Rupture (MOR) of from about 3500 psi to about 6500 psi at
2550.degree. F. (1400.degree. C.) in N.sub.2, depending on the size
of the alumina filler material; a thermal shock resistance
parameter (resistance to crack propagation, Rst) of about
60.degree. F./in. 1/2; a volume stability (thermal expansion in
accordance with ASTM E228.71 from room temperature to 1500.degree.
C. and then cooled) of 0.15% or less in linear change with no rate
changes that result in cracking or distortion; and a corrosion
resistance (air/metal line wear in inches with a major diagonal
1.times.1 inch bar, 20 min. spin test, Al-killed steel, as
described in the example below) of 0.04 inch or less.
The ceramic composite of this invention exhibits substantially
clean grain boundaries wherein the grain boundaries at the
interconnection of the crystallites have no other phase present.
Most notably, the grain boundaries are devoid of any siliceous
phase. This feature is particularly important for steel plant
refractories. Low-melting silicates are found in almost every
traditional alumina refractory, and this material reacts with the
molten iron causing dissolution into the liquid steel and
ultimately leading to cracking, spalling and failure of the
structure.
In addition, the composites of the present invention do not require
extra precautions to prevent oxidation of the bonding phase because
it is a fully oxidized matrix, which is in contrast to
carbon-bonded alumina refractories presently being used in Japan in
the slide gate market.
A particularly effective method for practicing this invention
involves forming the filler into a preform with a shape
corresponding to the desired geometry of the final composite
product. The preform may be prepared by any of a wide range of
conventional ceramic body formation methods (such as uniaxial
pressing, isostatic pressing, slip casting, sedimentation casting,
tape casting, injection molding, filament winding for fibrous
materials, etc.) depending largely on the characteristics of the
filler. Initial binding of the particles prior to infiltration may
be obtained through light sintering or by use of various organic or
inorganic binder materials which do not interfere with the process
or contribute undesirable by-products to the finished material. The
preform is manufactured to have sufficient shape integrity and
green strength, and should be permeable to the transport of
oxidation reaction product, preferably having a porosity of between
about 5 and 90% by volume and more preferably between about 25 and
50% by volume. Also, an admixture of filler materials and mesh
sizes may be used. The preform is then contacted with molten parent
metal on one or more of its surfaces for a time sufficient to
complete growth and infiltration of the preform to its surface
boundaries.
As disclosed in copending U.S. patent application Ser. No. 861,024,
filed on May 8, 1986, in the names of Marc S. Newkirk et al and
entitled "Method of Making Shaped Ceramic Composites with the Use
of a Barrier" and assigned to the same owner, a barrier means may
be used in conjunction with the filler material or preform to
inhibit growth or development of the oxidation reaction product
beyond the barrier. After the first heat treating step and before
the second heating step, the barrier is removed by any suitable
means. Suitable barriers may be any material, compound, element,
composition, or the like, which, under the process condition of
this invention, maintains some integrity, is not volatile, and
preferably is permeable to the vapor-phase oxidant while being
capable of locally inhibiting, poisoning, stopping, interfering
with, preventing, or the like, continued growth of oxidation
reaction product. Suitable barriers for use with aluminum parent
metal include calcium sulfate (plaster of paris), calcium silicate,
and Portland cement, and mixtures thereof, which typically are
applied as a slurry or paste to the surface of the filler material.
A preferred barrier comprises a 50/50 admixture of plaster of paris
and calcium silicate. These barrier means also may include a
suitable combustible or volatile material that is eliminated on
heating, or a material which decomposes on heating, in order to
increase the porosity and permeability of the barrier means. The
barrier is readily removed from the composite as by grit blasting,
grinding, etc.
As a result of using a preform, especially in combination with a
barrier means, a net shape is achieved, thus minimizing or
eliminating expensive final machining or grinding operations.
As a further embodiment of the invention and as explained in the
Commonly Owned Patent Applications, and patent the addition of
dopant materials in conjunction with the parent metal can favorably
influence the oxidation reaction process. The function or functions
of the dopant material can depend upon a number of factors other
than the dopant material itself. These factors include, for
example, the particular parent metal, the end product desired, the
particular combination of dopants when two or more dopants are
used, the use of an externally applied dopant in combination with
an alloyed dopant, the concentration of the dopant, the oxidizing
environment, and the process conditions. The dopant(s) used in the
process should be substantially removed or oxidized during the
second heating step so as to not adversely affect the properties of
the end product.
The dopant or dopants used in conjunction with the parent metal (1)
may be provided as alloying constituents of the parent metal, (2)
may be applied to at least a portion of the surface of the parent
metal, or (3) may be applied to the filler bed or preform or to a
part thereof, or any combination of two or more of techniques (1),
(2) and (3) may be employed. For example, an alloyed dopant may be
used in combination with an externally applied dopant. In the case
of technique (3), where a dopant or dopants are applied to the
filler bed or preform, the application may be accomplished in any
suitable manner, such as by dispersing the dopants throughout part
or the entire mass of the preform as coatings or in particulate
form, preferably including at least a portion of the preform
adjacent to the parent metal. For example, silica admixed with an
alumina bedding is particularly useful for aluminum parent metal
oxidized in air. Application of any of the dopants to the preform
may also be accomplished by applying a layer of one or more dopant
materials to and within the preform, including any of its internal
openings, interstices, passageways, intervening spaces, or the
like, that render it permeable.
The invention is further illustrated by the following example.
EXAMPLE
Aluminum Association 712.2 aluminum casting alloy ingot measuring 1
inch by 21/2 inches by 81/4 inches was placed horizontally upon a
layer of a mixture of commercial 8-14 grit pure alumina (Norton
Co., 38 Alundum) and 5 weight percent 500-mesh SiO.sub.2
(Pennsylvania Glass and Sand Co.) and was subsequently covered with
the same material to a depth of approximately three inches. The
712.2 alloy comprised, by weight percent, about 5 to 6.5% zinc,
about 0.25% or less copper, about 0.4% to 0.6% chromium, about
0.15% or less silicon, about 0.40% or less iron, about 0.25% or
less to 0.50% magnesium, about 0.10% or less manganese, about 0.15%
to 0.25% titanium, about 0.20% or less of other metals with the
maximum amount of any one other metal being about 0.05% or less,
and the balance being aluminum.
The alumina-embedded ingot was contained within a suitable
refractory crucible and the entire assembly was placed into an air
atmosphere furnace. The furnace allowed the entry of ambient air
through natural convection and diffusion through random openings in
the furnace walls. The assembly was processed for 144 hours at a
setpoint temperature of 1000.degree. C. after allowing an initial
eight-hour period for the furnace to reach the setpoint
temperature. After the 144 hour heating period, eight additional
hours were allowed for the sample to cool to below 600.degree. C.,
after which the resulting ceramic composite was removed from the
furnace. The ceramic composite contained residual zinc, aluminum
and silicon.
In order to remove at least a substantial portion of the residual
zinc, aluminum, and silicon, the ceramic composite was again
contained within a refractory crucible, placed into the air
furnace, and was processed for eight hours at a setpoint
temperature of 1400.degree. C. after allowing an initial eight-hour
period for the furnace to reach the setpoint temperature. After the
eight-hour heating period, eight additional hours were allowed for
the ceramic composite to cool to below 600.degree. C., after which
the ceramic composite was removed from the furnace. The alumina
matrix changed from a gray, metallic color to a white color after
the second heating step of 1400.degree. C., indicating very little
presence of residual metal. The microstructure of the ceramic
composite revealed a very homogeneous, porous, fine-grained
(approximately 6 micron diameter) alumina matrix. The residual zinc
volatilized, effectively driving off any residual aluminum and
silicon and providing space for in situ oxidation of some of the
aluminum during the second heating step at 1400.degree. C.,
ultimately creating a more porous, low metal content ceramic
composite. The second heating step at 1400.degree. C. caused no
further substantial oxidation reaction product growth beyond the
original defined boundary of the composite, even though aluminum,
zinc, and silicon metals were present prior to a second heating at
1400.degree. C. Bend testing showed a MOR (room temperature) of
approximately 4000 psi for the final composite, and a strength
retention (MOR) of about 2400 psi after five rapid heat-up and
cool-down cycles between room temperature and 1200.degree. C. with
ten-minute soak periods at each temperature. X-ray analysis of the
ceramic product showed alumina and some minor amounts of zinc
aluminate.
To examine the effect of molten steel on this ceramic product, the
ceramic product was cut into four pieces and engaged to four sample
holders threaded to a bearing-supported shaft of a spin test
apparatus consisting of a steel frame holding a variable speed
electric motor connected to the bearing-supported shaft. The four
pieces of ceramic product were rotated with the sample holders
about the central axis of the bearing-supported shaft. The outer
edge of each of the ceramic product pieces traveled at 600 inches
per minute when rotated at 48 rpm. A sheet grade steel (low carbon,
sulfur, phosphorus, and oxygen) was heated to 1593.degree. C. and
the surface deslagged prior to the start of the test. The four
pieces of ceramic product were heated to 1093.degree. C. and then
immersed in the molten steel and rotated at 48 rpm by the spin test
apparatus for 20 minutes. The four pieces of ceramic product were
removed from the sample holders, cooled, and examined to determine
the effect of molten steel upon the ceramic product. It was
determined that the ceramic product resisted significant
penetration of steel, did not react to any extent with the liquid
steel, and did not fracture during the test due to any thermal
gradients. Thus, the ceramic composite product appears to be a
useful steel refractory, such as for slide gate valves that are in
contact with molten steel.
* * * * *